I think you are correct if you are gearing by class, but I think that gearing by role might be a bit different. Without the book upgrade dropping with every clear, I don't think it would speed up gearing significantly. It would certainly be slower than if you got a book drop and two coffers. I simply think that if you can tank in one raid group, DPS in another, and heal in another, all at a high enough level to clear, you should be permitted to without impacting the coffer chances. It's possible someone would be able to do that, get lucky enough to have only one role's items drop in multiple raids, and win the rolls for them, as you say, and that would increase gearing speed for that role. I don't think that would be likely, though, as that means your other clearing raids would need to deal with someone whose gear is not increasing in ilvl (because they wouldn't be applying gear to any other roles), and you'd need the other raid members to not roll on them (because if you're trying to, say, gear a healer with multiple items you can run as a healer once, then you're running as a tank or DPS and can't need roll, a situation unlikely until later in the patch cycle).

I do understand your point, and making sure you couldn't do that by locking the individual loot by role would be time consuming, but I don't think it would be necessary. I also think it might make it a little easier to fill out raids.